Abstract:
The Southern Oscillation Index (SOI) at NCAR is computed using monthly mean sea level pressures at Tahiti (T) and Darwin (D). The SOI [T-D] is an optimal index that combines the Southern Oscillation into one series. These SOI values are slightly different than those calculated by the Climate Prediction Center (http://www.cpc.ncep.noaa.gov/) due to the normalization used. The [T+D] series is a measure of small scale and/or transient phenomena that are not part of the large scale Southern Oscillation.

Quality
The SOI values prior to 1935 should be used with caution. There are questions regarding the consistency and quality of the Tahiti pressure values prior to 1935. Perhaps a better estimate of the SOI prior to 1935 is the normalized Darwin pressure anomaly series.